1. Prior Art
U.S. Pat. No. 3,983,401 entitled "Method and Apparatus for Target Support in Electron Projection Systems" by W. R. Livesay dated Sept. 28, 1976.
An article entitled "Electrostatic Wafer Chuck for Electron Beam Microfabrication" by George Wardly, Rev. Sci. Instrum., Vol. 44, No. 10, October, 1973 (cited in the Livesay Patent, supra).
U.S. Pat. No. 3,900,737 entitled "Electron Beam Exposure System" by Collier, et al., dated Aug. 19, 1975.
U.S. Pat. No. 3,801,792 entitled "Electron Beam Apparatus" by Lin dated Apr. 2, 1975.
U.S. Pat. No. 4,189,230 entitled "Wafer Holder with Spring-Loaded Wafer Holding Means" by Zasio, dated Feb. 19, 1980.
2. Field of the Invention
This invention relates, in general, to a new and improved apparatus (wafer cassette) for supporting and securing target wafers for exposure to a particle beam in particle beam lithographic systems.
The above Wardly article explained the technique of using an electrostatic chuck in microfabrication applications using Coulombic forces to support and flatten a target wafer. The wafer was supported on a backing plate electrode with an insulator (dielectric) therebetween with a potential difference applied to the wafer and to the backing plate electrode.
The above Patent to Livesay explained at length the problems involved in supporting and securing target wafers in electron image projection systems and in electron pattern generator systems utilizing an electron beam deflection. For further explanation of the operation of electron beam pattern generating systems, see also the Patents to Collier, et al. and Lin, supra.
The chuck of the Livesay Patent, like the chuck of the Wardly article, placed a wafer on a dielectric which in turn is supported on a flat wafer support and applied a bias voltage across the dielectric layer to flatten the wafer. The target wafer is in an electron accelerating electric field. Also disclosed is a wafer support means which included a lip extending over the edges of the wafer for minimizing the electric field distortion due to the presence of the target support in an electron accelerating field in an image projection system. This latter wafer support, however, did not use Coulombic forces to flatten the wafer.
The Zasio Patent illustrated a wafer holder which spring loaded the wafer against a flange (mechanical stop) and several means to index (align) the wafer in the holder but did not use the Coulombic force concept to flatten the wafer.
Notwithstanding the prior art, there still exists a need for electrostatic chuck by which a wafer is held both mechanically and electrostatically which includes means by which the wafer can be removed and replaced within a very close tolerance for the numerous steps in the process of fabrication of integrated circuits and which includes means for positively forming a good electrical connection to the wafer.